So far, everything works pretty well, I stopped extensively testing it and now the router runs in ‘every day work mode’ meaning there are 2-4 wireless and 1 wired client connected, none of them consumes a lot of bandwidth, just typical usage. I checked the log, the router did not restart for several days. The router runs switches periodically from/to repeater / wired mode, AdGuard uses 10 host files, the screenshot is attached.
This feature is the one I waited/wanted for the most.
I installed several additional packages such as the frond end for MWAN3 and made some changes in the config (which probably means that I slightly changed the original GL config).
I tried to tether a smartphone, it worked to some extended, meaning it started working fine and stop a bit later. To achieve that I had to modify TTL and hope limit counter for all interfaces.
But there is something that bothers me - when I capture the packets leaving the WAN, I see different TTL and this is very strange (although it has been reported by another user once).
The packets with incorrect TTL coming from MWAN3 which uses ping for check the state of specific interfaces. And the custom setting of TTL for POSTROUTING does not affect the TTL of the packets sent by MWAN3.
This is something I want to look into later, I also use GL-AR750S-Ext / Slate, running OpenWrt 18.06 with MWAN3 (and such) and in that case TTL is set correctly, the tethering works without any glitches.
This is probably all what I can say about AXT1800 right now.
I am not sure how I would compare Beryl with AXT1800. I would say that Beryl has a bit stronger signal than GL-AR750S, more memory, faster WiFi and a bit faster CPU. The best thing about Beryl is that it runs well OpenWrt while AXT1800 cannot (and this is important for me). The best thing about AXT180 that it has even more memory, faster CPU, faster WiFi and it can run AdGuard (and this is important for me). That is why I have both. But for you it is probably better just wait for a little longer before you can decide which router to get.
Plus, it would be useful to know what are your priorities in this regard:
a) run original OpenWrt;
b) run DNS filtering (AdGuard);
c) and so on.